Sandia National Laboratories (SNL) is developing a cooling technology concept – the Sandia National Laboratories Natural Circulation Cooler (SNLNCC) – that has potential to greatly improve the economic viability of hybrid cooling for power plants. The SNLNCC is a patented technology that holds promise for improved dry heat rejection capabilities when compared to currently available technologies. The cooler itself is a dry heat rejection device, but is conceptualized here as a heat exchanger used in conjunction with a wet cooling tower, creating a hybrid cooling system for a thermoelectric power plant.The SNLNCC seeks to improve on currently available technologies by replacing the two-phase refrigerant currently used with either a supercritical fluid – such as supercritical CO2 (sCO2) – or a zeotropic mixture of refrigerants. In either case, the heat being rejected by the water to the SNLNCC would be transferred over a range of temperatures, instead of at a single temperature as it is in the TSC. This has the potential to improve economic dry heat rejection performance in three ways: decreasing the minimum temperature to which the water can be cooled, increasing the temperature to which air can be heated, and increasing the fraction of the year in which dry cooling is economically viable.SNL demonstrated that sCO2 can be used in a natural circulation device to reject heat in 2014 and 2015 at operating temperatures near the critical point of CO2 ~ 88˚F. This means that stable natural circulation of sCO2 can be achieved in a range of interest for some power plants. However, establishing and maintaining natural circulation for temperatures far above the critical point has yet to be demonstrated. At pressures above the critical pressure (1071 psi), there are pseudo-critical points that indicate the most likely temperatures and densities about which natural circulation is easily maintained. Pseudo-critical points can be visualized on a Temperature-Density plot as inflection points on a given isobar. As the pressure increases above the critical pressure, the temperature corresponding to the pseudo-critical point also rises, indicating that economical dry heat rejection is possible at temperatures that are significantly higher than the critical temperature. Should cost-effective and safe dry heat rejection be demonstrated at ambient air temperatures nearing 100˚F, the effects on power production in hot, arid climates would be profound.Developing a supercritical fluid with a higher critical point (at or above 100˚F) and favorable properties for the SNLNCC, or a zeotropic refrigerant mixture that can operate at or above 100˚F remains an important goal, as it would guarantee a means of saving water in very hot, arid climates where water availability is often limited.

The Sandia National Laboratories Natural Circulation Cooler

Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2021/volume/85109

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